STEEILITY 191 



ring. In other words, there is here a concrete illustration 

 of the primaiy importance of reproduction in all evolu- 

 tion. Since provision for succession exceeds all other 

 matters in import to the species, new variations are con- 

 stantly taking place, new processes are continuously being 

 tried out. The result is to have reproduction tied up with 

 more complications than any other physiological process, 

 to have in naturally cross-bred species more heterozygous 

 factors than in any other character complex. Repro- 

 duction, therefore, as we have seen, is affected more 

 often and more frequently than anything else when 

 inbreeding occurs. 



When inbred strains showing reduced fertility are 

 crossed, on the other hand, there is almost always a return 

 to the original productiveness along with the return to the 

 original size and vigor. In fact, just as fertility is 

 affected adversely by inbreeding more than any other 

 character, so is it increased more in proportion by cross- 

 ing. But such increases in productiveness are the rule 

 only up to a certain point of germinal difference between 

 the individuals taking part in the cross. As dissimilarity 

 in the uniting germ plasms becomes greater, sterility 

 again manifests itself. This time, however, the sterility 

 shown is of a different nature. No structural abnormali- 

 ties appear. There are no variations such as are found in 

 the numerous strains differentiated by inbreeding. It is 

 simply a matter of non-production of functional gametes. 



Based upon these germinal differences crosses be- 

 tween species may be classified arbitrarily as follows : 



(1) The hybrid may have the same or greater vigor 

 and fertility than the parents. Nicotiana alata and N. 

 Langsdorffii, for example, are distinct species having dif- 



